Elastic wave filters that are drawing attention as high-frequency filters, such as an SAW filter utilizing surface acoustic waves and a BAW filter utilizing bulk acoustic waves, for example, were large in size, and were therefore typically mounted on the top surface of a substrate (see Patent Document 1 below). However, in recent years, along with the reduction in size of the elastic wave filters, which was made possible by advancement of the wafer level package technology, development of a technology to incorporate the elastic wave filter in the substrate in the same manner as small electronic components such as capacitors, inductors, and resistors has been pursued.
However, because a typical elastic wave filter is configured such that a filter function section thereof is covered by a hollow cover, the following problem may arise when the elastic wave filter is built into the substrate.
When the elastic wave filter is built into the substrate such that the hollow cover faces up, for example, the substrate with a built-in electronic component has at least an insulating layer above the elastic wave filter. On the top surface of this substrate with a built-in electronic component, large electronic components such as an IC chip are mounted as needed, in addition to small electronic components. When an external force is applied downwardly to the top surface of the substrate with a built-in electronic component during, before, or after such a mounting step or the like, downward stress resulting from the external force acts on the hollow cover of the elastic wave filter. As a result, the hollow cover may be pressed and deformed, which may cause functional anomaly, damage, and the like in the filter function section.